power: reset: make function sc27xx_poweroff_shutdown static

The function sc27xx_poweroff_shutdown is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
drivers/power/reset/sc27xx-poweroff.c:28:6: warning: symbol
'sc27xx_poweroff_shutdown' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>